CSS ID Selector

What is a CSS ID Selector?

• The ID selector specifies an element whose id attribute value is equal to a specific value. This selector is specified by preceding the id attribute value with a # symbol. If the # symbol is immediately preceded by a selector then the style will only be applied to a matching element that also has a matching id attribute value. In the following example, both of the style declarations apply to the <h1> element.
• Note: An ID attribute uniquely identifies an element. Therefore, it is not valid for multiple elements to have the same ID attribute value.
  

CSS Example:

  #tst1 {text-align: center;}
  h1#tst1 {color: red;}

  <h1 id="tst1">Both style declarations apply.</h1>